The manufacture of two-piece golf balls, i.e., golf balls with only a solid core ball and a dimpled outer shell, has normally been done in an injection molding process. In this process, the solid core is first placed inside of a mold cavity, a set of pins then pushes upward through the bottom of the cavity to raise the core to a central position, and a molten material is then forced into the cavity and around the core to form the outer shell.
Although this process makes well-formed balls, it is a difficult one to control and requires skilled operators. Also, it requires expensive tooling and expensive maintenance to the tooling. Ideally, it would be preferred if a less expensive process could be found.
While manufacturers have had to use injection molding to form the dimpled shell on two-piece balls, they have been able to use a much less expensive process of compression molding for forming a similar shell on three-piece balls, i.e., the traditional golf ball with a small rubber core ball surrounded by a mantle or intermediate layer and then the outer shell. Obviously, the manufacturers would prefer to use compression molding in forming two piece balls because the equipment and tooling are already available in their plants. Further, the general process of producing the outer shell would be less expensive because it would be easier to control and would therefore not require expensive, highly skilled operators.
Unfortunately, attempts to compression mold perfectly shaped, two-piece balls have failed because, unlike the wound centers of three-piece balls, the solid cores of two-piece balls are relatively incompressible. In 1902, Kempshall attempted to solve this problem by using telescopically mating dies to compression mold a dimpled cover for two-piece balls. As described in the Kempshall U.S. Pat. No. 695,867, he teaches a process of compression molding includes a pair of male and female dies. As shown in FIGS. 5-7 of the Kempshall patent, the female die has a rim or step C on which a small amount of the cover material extrudes when the dies are initially pushed together to form the cover. The overflow material is trapped between the approaching edges B and C of the two dies. Continued axial pressure on the dies causes the edges B and C to move toward one another and force the trapped material to flow back inside the mold cavity to create an integral dimpled cover for the ball.
While the Kempshall attempt was noteworthy, it has its drawbacks. As explained in the patent, the produced cover is not uniform because it has a welt at its equator. This welt prevents the ball from being consistently hit the same distance with any single golf club by an accomplished golfer. Since predictability in the length of a “shot” is important for a low score in golf, this lack of consistency and the welt which causes it are highly undesirable.
The inconsistency occurs when the “strike” of the golfer's club head hits against the welt. Because this portion of the cover is fatter than the rest, the strike creates less compression of the ball than it would if any other portion of the cover were struck. This lesser compression causes the ball to go a shorter distance than it would normally travel.
In addition to creating a welt, the compression molding process can allow the size of the welt to vary from ball to ball. Since the volume of the solid core for two-piece balls can vary substantially from one core to the next, it is difficult to precisely control the amount of cover stock extruded from the mold cavity during mating of the dies. With these molds, if a large core were encountered, an excessive amount of cover stock would be extruded and the same large amount would be forced back into the mold cavity with the result being an even larger deformation or welt at the center of the finished ball cover. Also, wear would occur on the approaching edges B and C of the dies because of the extra pressure placed on the edges by the increased bulk between them.
It is therefore the primary object of the present invention to provide an improved method for placing covers over two-piece golf balls by compression molding.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.